In connection to the compressing of a powder to a green body during production of cutting inserts, the powder is introduced into a cavity defined by a die. Normally, the die comprises an upper opening through which the powder is introduced into said cavity and through which, during a subsequent pressing step, an upper punch is introduced into the die. Typically, there is also provided a lower punch which is able of sliding through a tunnel in the die and which will form at least part of a bottom of said die cavity and by means of which the green body formed upon compression of the powder is ejected from the surrounding die. From the upper opening of the die there is provided a punch tunnel in which the upper punch is able to move downwards for the purpose of contacting the powder and subjecting it to a compacting pressure. In other words, the punch tunnels define the die cavity provided for receipt of the powder, and the punches are provided for the purpose of compacting a powder received in said cavity.
After compaction of the powder to a green body, the upper punch is retracted out of the die, and the green body is ejected by a motion of the lower punch relative to the die (either of these components could be the one which is moving). Accordingly, the green body is ejected through the tunnel in which the upper punch was moving downwards into the die during the pressing step.
Typically, the material of the green body is of such nature that it will expand when ejected and released from the surrounding die. In order to enable the green body to expand radially when it is ejected from the cavity in which it has been compressed, the tunnel is widened slightly above the level to which the upper punch is forwarded during compaction. It could be said that the cavity is provided with a release portion having a certain inclination angle of the inner wall of the die relative a centre axis of the cavity. The inclination angle (possibly also referred to as the release angle), as well as the length (in the vertical direction) of the release portion is adapted to the expected (radial) expansion of the green body upon ejection thereof.
Typically, the cavity has a cross section that narrows as seen from the release portion to the remaining (lower) part of the cavity. Upon compression of the powder, a lower edge of the upper punch, which is defined by the intersection of a lateral surface and a bottom surface thereof, is not allowed to come into contact with the inner peripheral surface of the die, since such contact might result in damages on both die and punch. Therefore, the punch is only forwarded to a level at which there will be a small gap between said punch edge and the inner peripheral surface of the die. During compaction of the powder, the latter will be able of leaking out through said gap and into the release portion. Such leakage will result in a residual edge being formed on the green body along the upper edge thereof, which needs to be treated, i.e. removed, before the subsequent sintering of the green body. Such treatment is time-consuming and contributes to unwanted production costs. The leakage also results in unwanted loss of material. Further negative effects of powder leakage may also be an unwanted effect on the shape of the upper edge of the green body or a lower density, i.e. a generation of porosity, in the region of the upper edge of the green body.